In many applications control circuitry used to control an incandescent lamp or other load which has a nonlinear impedance during turn-on or warm-up comprises a semiconductor device such as a power transistor (e.g., an n-channel Metal-Oxide-Silicon Field Effect Transistor (MOSFET)). A single power n-channel MOSFET can be connected with its drain connected to a power supply having a positive voltage +VDD and its source connected to one terminal of an incandescent lamp. A second terminal of the lamp is connected to ground potential. The gate of the MOSFET is typically coupled through a switch, in many applications a hand operated switch, to a positive voltage source which typically has a potential level of +VDD or in many cases +2VDD. When the switch is turned on the MOSFET is enabled (turned on) and a current path is created from +VDD, through the MOSFET and lamp, and then to ground. The electrical characteristics of an incandescent lamp are such that while the lamp is cold its resistance (the "cold" resistance) is relatively low. As the lamp starts to conduct it heats up and its resistance increases in a nonlinear manner with its "hot" resistance being significantly greater than its "cold" resistance. The MOSFET used must be designed to be able to dissipate a relatively high amount of power as the lamp is turning on since there is a large current spike generated and a significant portion of the voltage of the power supply is across the drain-source of the MOSFET during turn on since the lamp's "cold" resistance is relatively low.
FIG. 1 shows a graph of lamp current in amperes on the y-axis versus time (t) in milliseconds (mS) on the x-axis for a lamp which when fully on and operating in steady state draws about 6 amperes and has a resistance of about 2.25 ohms. Between t=0+ and t=2mS the lamp draws 42 amperes of peak current which decreases such that at about t=40mS a steady state current of 6 amperes is reached. FIG. 2 shows a graph of the junction temperature in degrees C. on the y-axis of a MOSFET in series with the lamp versus time (t) in milliseconds (mS) on the x-axis. The temperature of the junction rapidly increases from 80 degrees C at t=0 to approximately 205 degrees C at t=9mS and then decreases to 95 degrees C by t=40mS. The thermal constant of the MOSFET determines the rate of rise of the temperature thereof The maximum desirable temperature of many MOSFETS is approximately 160 degrees C. Thus under the above described conditions the MOSFET can be damaged. One solution to this problem is to increase the area of the MOSFET such that it can handle 42 amperes of peak current without its temperature exceeding 160 degrees C. This solution is undesirable from an economic view point since a larger area MOSFET requires more silicon and is therefore more expensive to produce.
One prior art solution to the junction temperature problem is to pulse the potential of the gate of the power MOSFET such that it is on for only a short period during each cycle. This limits the average current during a cycle and moderates the temperature rise of the MOSFET. One problem with this technique, which is sometimes denoted PWM (Pulse Width Modulation), is that a plurality of relatively large current spikes are generated which can easily be coupled as noise to other circuits using the same power supply. This is very undesirable in some applications such as in an automobile where turning on of the directional signals could cause multiple loud noises on the radio or tape playing system.
It is desirable to be able to turn on a load whose impedance increases in a nonlinear manner during turn-on using moderate cost control circuitry which includes a power transistor without causing repetitive large noise signals.